Mehran rear suspension system alteration. help required?

Dear mehran owners, I want to change the shaky rear kamanis system(leaf spring) of mehran to coil spring system as you can see in Jiangnan mehran(chinese copy of mehran) (pic. 1&2). Can you guys tell me the part of chassis in our pakistani mehran(pic. 3) that holds the front end of kamanis is strong enough to hold the end of control arm? (pic. 4 & 5 can show you the control arm). Or any chassis modification is required ? as for control arm only one end gets fixed compared to the kamanis whose both ends are fixed.
Also what is the purpose of this diagnal rod shown with green in pic.5?

bro the mehran (Alto) that was launched all over the world came with two variety
1)base model with leaf spring (same as our mehran)
2)upper model that come with proper suspension
the latest JDM alto g2 come with coil spring if you can find its suspension I think it will fit in mehran